In color printing machines, a printed image is created on a printing material by superimposing several color separations. If the color separations are not exactly registered on the printing material, these deviations are well-visible in the printed image and hence represent a quality defect.
The color printing machine may be an offset printing machine, in which the color separations are created by different printing plates. To accomplish this, each of the colors is assigned its own type form, e.g., a printing plate.
The color printing machine, however, may also be an ectrophotographic printing machine. This printing machine may comprise a separate printing unit for each color separation. By using imaging devices, which may include, for example, lines (rows) or matrices (arrays) of laser diodes or LEDs, a latent image is generated on an imaging web or on an imaging cylinder. Depending on the printing unit, this image is associated with a specific color.
Toner of the appropriate color may be applied to the imaging cylinder. The resultant toner image corresponds to the latent image on the surface of the imaging cylinder.
Depending on the type of printing machine, the design may provide that the toner adheres only to the exposed areas of the printing cylinder or only to the unexposed areas of the printing cylinder.
Depending on the type of printing machine, the toner may be applied directly from the imaging cylinder to the printing material, e.g., paper, or this step may take place by means of an interposed transfer material means, a rubber blanket cylinder or a transfer belt. In so doing, a corresponding color separation is created on the printing material.
Furthermore, printing machines have been known, in which one printing unit produces each color separation. It is also possible that various color separations are first printed on top of each other on a transfer means, such as a transfer belt, and are then transferred from this transfer means to the printing material in one step.
Depending on the alignment of the imaging devices or depending on the adjustment of their respective zero positions, or even other error sources not mentioned here, it may happen that individual color separations are not superimposed exactly, not registered accurately, on the printing material, as would be desirable for the resultant printed image. These positional deviations of the color separations from their set positions may occur, in particular, in a direction perpendicular to the transport direction of the printing material. Hereinafter, this direction will also be referred to as the scanning direction or X-direction, because this is a direction which can be associated with the advance direction, i.e., the scanning direction of an imaging device. The imaging device scans the surface of a printing plate or an imaging cylinder in axial direction, while said plate or cylinder rotate under it, thereby permitting the desired printing image to be applied to their surfaces.
In order to avoid quality reductions of the printed image caused, in particular, in scanning direction due to the positional deviation of the color separations, for example, U.S. Pat. No. 5,384,592 to Wong has disclosed the application of register marks on the printing material.
U.S. Pat. No. 5,384,592 to Wong suggests two alignment indicators consisting of two register marks comprising lines that are inclined parallel to each other, and in which case each register mark has a front line and a rear line extending perpendicularly in transport direction. In so doing, the register marks are applied in the shape of triangles. A sensor measures the times at which the front line and the rear line, respectively, and the two parallel inclined lines pass through the sensor. The difference between the measured times is then used to compute the focus of the alignment indicators. With the knowledge of the measured foci of the alignment indicators for various color separations, deviations of the color separations can be corrected. In particular, the alignment indicators described in this reference provide that two alignment indicators are always created in one line perpendicular to the transport direction of the printing material on different sides of an image zone of the printing material. This is intended to also allow the detection of position errors of the color separations in the direction of the transport path of the printing material. Essential elements of the method described in this reference are that the foci of the alignment indicators must be detected; therefore, a uniform transport speed of the printing material must be ensured. Irregularities in the transport speed can result in measurement errors. Furthermore, in particular, lines of alignment indicators perpendicular to the transport direction and diagonal parallel lines that are inclined relative to the former are necessary. Poor positioning of the imaging devices, which could result in inclined positions of the alignment indicators, can result in errors such as unwanted enlargements of individual picture elements caused, e.g., by optical element errors, when the alignment indicators are analyzed. If an enlargement occurs, for example, the parallel arrangement of the inclined lines is no longer absolutely given.